Light scanning units generally scan laser beams on a photosensitive drum in an image forming apparatus such as a laser printer, a copying machine, a facsimile machine, or the like that reproduces images on printing media to form electrostatic latent images.
A light scanning unit may comprise a light source unit, a polygon mirror, and a detection unit for synchronization purposes to obtain horizontal synchronization signals of light that is scanned due to the rotation of the polygon mirror. Particularly, in a tandem light scanning unit, which is used in a color image forming apparatus, two laser beams may be emitted from the light source unit and deflected by a polygon mirror. Thus, when two laser beams are deflected by a common polygon mirror, due to being affected by the arrangement of the synchronization detecting unit, the starting position of scanning lines may be unstable due to error in spacing angles of the polygon mirror, or a complex optical system of the detection unit for synchronization of the laser beams may be required. Therefore, it is desirable to design an improved light scanning unit that may be incorporated into an image forming apparatus.